Detection and avoidance of obstacles are very important to the navigation of ground and air vehicles. A system which can provide autonomous obstacle detection and avoidance is needed for such vehicles. The development of an autonomous system can be achieved by the use of active sensors (millimeter wave (MMW) radar, laser radar), passive sensors (television or forward looking infrared (FLIR)), or a combination of active and passive sensors.
An active system (MMW or laser) requires a very specialized and expensive sensor system. The active system risks detection by the enemy in a battle environment. Such system does not maximize usage of passive sensor technology.
Various active systems are most advantageous in certain kinds of environments. For all weather conditions, MMW radar is better suited than laser radar. However, for terrain following and avoidance, and obstacle detection and avoidance, laser radar is preferred because it is less susceptible to detection by the enemy and has the necessary resolution to detect wires (e.g., a 3 millimeter (mm) diameter wire at a 40 meter distance), while MMW radar operating at 94 gigahertz (GHz) having a wavelength about 3 mm, is marginally satisfactory. A laser sensor is also better than a MMW sensor for detecting objects like thin wires at oblique angles. For day/night operation and countermeasure resistance, both laser and MMW sensors are equally good. In view of the above trade-offs between MMW and a laser radar, a laser ranging system is preferable. However, many laser scanners are not adequate for such systems due to their slow scan rate and a lack of a large field of view (needed for providing a sufficient number of alternate directions of travel for a vehicle when an obstacle is encountered) for successful vehicle navigation.
Compared to active systems, a passive system has the benefit of covertness, simplicity, reduced cost and ease of manufacture. Obstacle detection using passive sensors permits the use of two fundamental techniques for ranging--binocular stereo and motion stereo (optical flow). With the binocular stereo technique, ranging performance is a function of the sensor resolution and the lateral displacement between the two sensors; increased displacement increases the maximum range measurement and improves range resolution. For vehicles, sensor displacement is limited by the dimension of the vehicle. The technique of motion stereo utilizes one sensor from which images are collected at regularly timed intervals while the sensor is in motion. By observing the amount of motion (on an image plane) that a world point exhibits between frames and using knowledge of sensor motion, range to the world point can be computed. The resolution of motion stereo techniques is limited only by the resolution of the sensor.